A serious problem in the use of high power electric arc etching tools or high current welders is that they tend to produce acoustic noise impulses of such high amplitude as to cause deafness in the operator of the equipment if the operator is exposed to such noise over long periods of time. The noise is caused by a sudden change in the electric current conditions of the plasma gas between the welding electrodes. When current is flowing steadily between the electrodes in normal welding operation, a force called the Lorentz force is produced that tends to constrict the plasma gas to a certain volume. When the current is turned off deliberately or when it is inadvertently interrupted, the Lorentz force disappears and the heated plasma gas is suddenly able to expand in volume. This sudden expansion constitutes the beginning of a shock wave whose outward propagation is registered in the ear of the operator or anyone else in the vicinity of the arc, as a loud impulse noise. During ignition of the arc, the creation of the Lorentz force results in a sudden collapse of the column of plasma gas through which the arc current flows, but this sudden collapse can only result in the outward propagation of an ordinary acoustic rarefaction wave whose intensity is far less severe than that of a shock wave. Therefore, the noise perceived as a result of this rarefaction wave, although unpleasant, is not as loud nor as dangerous as the noise produced when the arc is quenched.
In normal welding operation, these impulses occur sufficiently frequently to generate an average noise level that is likely to exceed standards established by the Occupational Health and Safety Acts of 1970 (commonly called OSHA). The current practice followed in order to meet the OSHA noise requirements is to limit the length of time a worker can be exposed to welding noise. The time commonly established is 1/2 hour per 8 hour day, which means that 16 welding machine operators would be required to keep one welding machine in continuous operation for an 8 hour day.